Summary: The use of a modified procedure for the isolation of covalently closed circular DNA of high molecular weight, followed by agarose gel electrophoresis of the crude extracts, provides a simple screening method for detecting plasmids with molecular weights of more than 250 × 10 from and species. This method was used for a survey of plasmids in 25 symbiotically effective strains of from various geographical origins. Of these, 22 strains were found to carry at least one large plasmid. By electron microscopy and measurement of electrophoretic mobility in gels, the molecular weights of most of the plasmids were estimated to range from 90 × 10 to 200 × 10.


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